1. Field of the Invention
This invention relates to pulse-position modulation in a spread-spectrum communication system.
2. Description of Related Art
In pulse-position modulation, a time delay between a first pulse and a second pulse is used to encode data. For example, a short time delay between the first and second pulses may indicate a logical "0", while a long time delay may indicate a logical "1". Successive time delays between successive pulses may be used to encode a stream of data bits. The receiver is able to determine the encoded stream of data bits by measuring the time delays between successive pulses.
In spread-spectrum modulation, a pseudo-random chip sequence is used to encode data. The resulting encoded signal is generally spread across a spectrum bandwidth which substantially exceeds the data-transfer rate, hence the term "spread-spectrum". The receiver produces a correlation pulse in response to the received spread-spectrum signal when it is able to match the chip sequence to a sufficient degree. Spread-spectrum communication often takes place in a noisy transmission environment.
One problem which has arisen in spread-spectrum communication is distinguishing those pulses which are produced by a true signal from those which are produced in response to noise, echoing, or other causes. One solution adopted by the assignee is to modulate each data bit with the entire chip sequence. This method is advantageous in reducing problems associated with synchronizing transmitter and receiver. However, to achieve a high data transmission rate, it requires a high transmission rate for the chip sequence.
Pulse-position modulation would appear well suited to a high data rate. However, in spread-spectrum communication, precise timing of the incoming signal can be degraded due to the requirement of generating a correlation pulse at the receiver. Moreover, in a system where each data bit is modulated with the entire chip sequence, delays between received pulses of less than the entire chip sequence in length would be difficult.
Accordingly, it would be advantageous to transmit data in a noisy environment, at a high data transmission rate, in which true pulses are easily distinguished from false, and in which synchronization between transmitter and receiver is easily achieved.